Beginning in 1986, there has been a resurgence in interest in superconductors. In that year, Bednorz and Muller discovered that the copper-based perovskite LaBaCuO showed a relatively high superconducting transition temperature T.sub.c. Since then, interest has shifted to YBaCuO with T.sub.c around 92.degree. K. The approximate chemical formula is Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x, with x being between 0 and 1, usually about 0.1 or 0.2 and indicating the oxygen stoichiometry upon which superconductivity critically depends. Closely related compounds ZBaCuO substitute Er, Gd, Eu or other elements for Y as the rare-earth component Z. These materials as presently grown have significant departures from stoichiometry because of imperfect crystallinity. Yet other classes of material offering yet higher transition temperatures of up to 125.degree. K. are represented by the formula A.sub.2 B.sub.2 Ca.sub.2n-1 Cu.sub.n O.sub.y where A is either Bi or Tl, B is either Ba or Sr, n=1, 2 or 3 and y is a value between 6 and 10. All of these high T.sub.c materials have been found to have a perovskite crystal structure. A perovskite crystal has a unit cell defined by lattice vectors a, b and c which are perpendicular to each other. In the high T.sub.c superconductors, a is equal or approximately equal in magnitude to b, but c is considerably larger. The location of the constituent atoms in a perovskite is described by A. F. Wells in his book entitled "Structural Inorganic Chemistry", 4th ed., Clarendon Press, 1975 at pages 149-154. In most of the high T.sub.c superconductors, the perovskite is in the orthorhombic form, that is, a.noteq.b, although they are relatively close. For most of these materials, as the oxygen deficiency is increased, the perovskite changes to the tetragonal form, that is, a=b, and the superconductivity disappears. However, there are some known tetragonal superconductors, such as YBa.sub.2 Cu.sub.3-x Co.sub.x O.sub.y for which x&gt;0.1.
It is of course anticipated that these high T.sub.c materials would be used in working electronic devices. However, to date, the results have been disappointing. Part of the reason has been the large and complex crystal structure involved in the high T.sub.c perovskite materials. While low T.sub.c Josephson junctions could be fabricated from superconducting niobium and its insulating oxide, the perovskites have not offered such simple material combinations. The high T.sub.c perovskites have a lattice constant of about 1.1 nm along the c-axis (the long perovskite axis) and of about 0.38 nm along the two axes in the a-b plane. However, superconducting coherence lengths .epsilon. for these high T.sub.c materials are of the order of 0.3 nm along the c-axis and of the order of 3 nm in the a-b plane. Therefore, the problem arises at the interface between different parts of the superconductive device which is not of the same superconductive quality as the bulk perovskite superconductor. For a multi-layer superconductive device, the interfaces must be sharp compared to the coherence lengths, that is, the transition of the T.sub. c of the film at the interface must occur over a distance which is less than the coherence length .epsilon.. For the Bi and Tl superconductors, the c-axis lengths are even longer.
Some researchers have reported Josephson effects in high T.sub.c materials. For example, the technical article by Hauser et al entitled "Response of YBaCuO thin-film microbridges to microwave irradiation" appearing in Applied Physics Letters, volume 54, 1989, pp. 1368-1370. However, most of these reports are based on intergrain effects. That is, the high T.sub.c material was granular with each of the grains being superconductive. However, the interface between the grains somehow provided the required non-superconductive material.
A vertical YBCuO Josephson junction device has been disclosed by Moreland et al in a technical article entitled "Evidence for the superconducting proximity effect in junctions between the surfaces of YBa.sub.2 Cu.sub.3 O.sub.x thin films" appearing in Applied Physics Letters, volume 54, 1989, pp. 1477-1479. According to this article, two separate YBaCuO films are fabricated and then squeezed together at their exposed surfaces. That is, the device is not monolithic.
Koch et al have described a YBaCuO SQUID (superconducting quantum interference device) in a technical article entitled "Quantum interference devices made from superconducting oxide thin films" appearing in Applied Physics Letters, vol. 51, 1987, pp. 200-202. In this device, either oxygen or arsenic was ion implanted into already formed film of YBa.sub.2 Cu.sub.3 O.sub.y to form a 17 .mu.m wide insulating strip between two superconducting YBaCuO regions. However, these devices showed low responsivity to magnetic fields. Further, the YBaCuO film had grain sizes of 2-5 .mu.m.
Geerk, Xi and Linker of Karlsruhe have disclosed in some recent unpublished work the fabrication of vertical supercondutive tunneling devices. By the use of reactive magnetron sputtering, a film of Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 was epitaxially grown on a SrTiO.sub.3 substrate and thereafter was oxidized at 450.degree. C. Sometime during the heat treatment or thereafter, a thin oxide formed on the YBaCuO. A counter electrode of Pb or In (superconductive at some of the operating temperatures) was applied by painting or attaching a wire. The asymmetry of the superconductive electrodes and the unknown insulator thickness made analysis of this device difficult.
In other recent and unpublished work, T. Shiota of Nagoya University has reported a symmetric YBaCuO tunnel junction. Both of the YBaCuO electrodes were deposited by RF magnetron sputtering. However, between depositions, the bottom electrode was plasmafluorinated to make a highly resistive thin barrier layer. The data reported to date by Shiota et al remain inconclusive as to the mechanism involved in the I-V characteristics of the YBaCuO-F:YBaCuOYBaCuO structure. Their results are difficult to interpret because of their admitted poor crystal quality.